1. Field of the Invention
The invention relates to a method for producing polymethacrylimide foamed materials as well as blocks, plates and the like of such polymethacrylimide foamed materials as well as the shaped articles obtained as intermediate products from the copolymer of methacrylic acid and methacrylonitrile.
2. Discussion of the Background
It is known how to produce polymethacrylimide foamed materials in the form of blocks. The first step is production of a preliminary product, which is already obtained in appropriate plate shape, from methacrylic acid and methacrylonitrile by copolymerization. The copolymer is then cyclized to the imide. An expanding agent present in the reaction mixture ensures appropriate foam formation when heated.
Heretofore, direct production of suitable polymer plates has been possible only up to a thickness of 30 mm. Since it is absolutely necessary to ensure a regular and reliable polymerization sequence, cooling and heating must be applied alternately during polymerization, especially because the polymerization can no longer be controlled and irregular structures are formed if heating is too intensive. These alternating cooling and heating phases are naturally cumbersome, and also entail high energy and water consumption.
In addition, polymer plates with thicknesses of only up to 30 mm suffer from numerous disadvantages. For example, if foamed blocks of relatively large thickness are needed, a plurality of foamed plates must be cemented together. Moreover, very large amounts of waste are produced from the edge regions during manufacture.
In German Patent No. 1817156 there is already described a method in which foamable plastics are produced in plate form by polymerizing mixtures of methacrylonitrile and methacrylic acid between two glass plates sealed with a flexible cord. An expanding agent such as formamide or monoalkylformamide is already added to the starting mixture. Radical sources are also added, in the form, for example, of a two-component mixture of tert-butyl perpivalate and benzoyl peroxide.
The polymerization takes place at temperatures of, for example, 40, 45 or 48xc2x0 C., and it lasts about 15 to 40 hours. The product is then tempered at about 100xc2x0 C. and subsequently heated to 170 to 300xc2x0 C. Cyclization to imide and foam formation take place at the latter temperatures.
It is difficult to ensure regular polymerization, because the temperature can very easily exceed the specified temperature. Temperature fluctuations must therefore be controlled very accurately and compensated for by alternating cooling or heating phases.
Plates thicker than 30 mm cannot be produced by this method, since temperature elevations occur repeatedly and, because of the relatively poor thermal conductivity of the polymerization mixture, the heat generated cannot be dissipated rapidly enough. Uncontrollable and unmanageable temperature elevations naturally lead to further increase of radical formation, and so the reaction very easily runs out of control.
A method similar to that of German Patent No. 1817156 is described in European Patent 0356714 A1. As the radical source there is used, for example, azobisisobutyronitrile, and 0.1 to 10 wt % of electrically conductive particles is added to the mixture to be polymerized.
The aforesaid problems also occur in this method, and the polymer plates obtained according to Example 1 of this European Patent Application have a thickness of only 25 mm.
Although numerous methods for producing foamed plates of polymethacrylimides are already known, a need still exists for improved methods of producing same and also for improved polymethacrylimide plates.
In particular, therefore, a need exists for thick foamed blocks and especially for a method with which such blocks can be produced without having to cement a plurality of blocks together.
The object of the invention is therefore to provide such blocks, especially in plate form, as well as a method for producing the same, with which a uniform block with thicknesses up to 80 mm and more can be produced in one step, which permits a reliable and controlled polymerization process and which operates with reduced water and energy consumption and which yields blocks in which relatively little waste is produced during manufacture. Another object of the invention is to provide an appropriate method in which the tempering bath, especially the tempering water bath, can be operated largely with constant temperature.
This object is achieved by a method for producing block-shaped polymethacrylimide foamed materials by copolymerization of methacrylic acid and methacrylonitrile as well as further copolymerizable monomers if necessary in the presence of radical-forming initiators, postpolymerization and cyclization of the copolymer to polyimide and transformation to a foamed material, which is characterized in that the copolymerization is performed in the presence of a mixture comprising at least three initiators with graduated half lives.
The subject matter is therefore a method according to the teaching of claim 1. Further advantageous embodiments are described in claims 2 to 7, and correspondingly produced blocks are described in claims 8 to 9. Further subject matter of the invention is methods according to claim 10, polymer plates according to claim 11 and the use according to claim 12.